I just purchased a couple sets of Maha Powerex NiMH 2500 mAh batteries. I tested them in a PMD660 against the regular alkalines I had been using and the increase was pretty good. With Energizer alkalines I was able to get 4:24 of record time and with the Powerex's I was able to get 7:04. I'm using a 4GB CF card, so at 16/44.1, I can fill up the card before having to change batteries. The 7:04 time was on the 2nd charge cycle, so I expect a slight increase on future charge cycles.
One odd thing about the PMD660 and rechargeables... with some rechargeables, the body is slightly longer and the positive post is slight shorter. The 660 battery compartment has little tolerance for this. When I first tried the Maha's, the 660 would NOT power on because the batteries weren't making contact (almost impossible to see). I ended up placing a paper shim behind each of the 4 positive contacts to get it to work.
Both tests were done with the PMD660, 16/44.1 stereo, with JW mod AKG 461's, phantom power on, attenuation on, normal meter lights on